In a conventional radiography system, an x-ray source is actuated to direct a divergent area beam of x-rays through a patient. A cassette containing an x-ray sensitive phosphor screen and light and x-ray sensitive film is positioned in the x-ray path on the side of the patient opposite the source. Radiation passing through the patient's body is attenuated in varying degrees in accordance with the various types of tissue through which the x-rays pass. The attenuated x-rays from the patient emerge in a pattern, and strike the phosphor screen, which in turn exposes the film. The x-ray film is processed to yield a visible image which can be interpreted by a radiologist as defining internal body structure and/or condition of the patient.
More recently, digital radiography techniques have been developed. In digital radiography, the source directs x-radiation through a patient's body to a detector in the beam path beyond the patient. The detector, by use of appropriate sensor means, responds to incident radiation to produce analog signals representing the sensed radiation image, which signals are converted to digital information and fed to a digital data processing unit. The data processing unit records and/or processes and enhances, the digital data. A display unit responds to the appropriate digital data representing the image to convert the digital information back into analog form and produce a visual display of the patient's internal body structure derived from the acquired image pattern of radiation emergent from the patient's body. The display system can be coupled directly to the digital data processing unit for substantially real time imaging, or can be fed stored digital data from digital storage means such as tapes or discs representing patient images from earlier studies.
Digital radiography includes radiographic techniques in which a thin fan beam of x-rays is used. In this technique, often called "scan (or slit) projection radiography" (SPR) a fan beam of x-rays is directed through a patient's body. The fan is scanned across the patient, or the patient is movably interposed between the fan beam x-ray source and an array of individual cellular detector segments which are aligned along an arcuate or linear path. Relative movement is effected between the source-detector arrangement and the patient's body, keeping the detector aligned with the beam, such that a large area of the patient's body is scanned by the fan beam of x-rays. Each of the detector segments produces analog signals indicating characteristics of the received x-rays.
These analog signals are digitized and fed to a data processing unit which operates on the data in a predetermined fashion to actuate display apparatus to produce a display image representing the internal structure and/or condition of the patient's body.
One of the advantages of digital radiography is that the digital image information generated from the emergent radiation pattern incident on the detector can be processed, more easily than analog data, in various ways to enhance certain aspects of the image, to make the image more readily intelligible and to display a wider range of anatomical attenuation differences.
An important technique for enhancing a digitally represented image is called "energy subtraction".
Energy subtraction exploits energy-related differences in attenuation properties of various types of tissue, such as soft tissue and bone, to derive a material-specific image, mapping substantially only a single material in the body.
It is known that different tissue, such as soft tissue (which is mostly water) and bone, exhibit different characteristics in their capabilities to attenuate x-radiation of differing energy levels.
It is also known that the capability of soft tissue to attentuate x-radiation is less dependent on the x-ray's energy level than is the capability of bone to attenuate x-rays. Soft tissue shows less change in attenuation capability with respect to energy than does bone.
This phenomenon enables performance of energy subtraction. In practicing that technique, pulses of x-rays having alternating higher and lower energy levels are directed through the patient's body. When a lower energy pulse is so generated, the detector and associated digital processing unit cooperate to acquire and store a set of digital data representing the image produced in response to the lower energy pulse. A very short time later, when the higher energy pulse is produced, the detector and digital processing unit again similarly cooperate to acquire and store a set of digital information representing the image produced by the higher energy pulse.
In early energy subtraction techniques, the values obtained representing the lower energy image were then simply subtracted from the values representing the higher energy image.
Since the attenuation of the lower energy x-rays by the soft tissue is about the same as the attenuation of the higher energy x-rays, subtraction of the lower energy image data from the higher energy image data approximately cancels out the information describing the configuration of the soft tissue. When this information has been so cancelled, substantially all that remains in the image is the representation of bone. In this manner, the contrast and visibility of the bone is substantially enhanced by energy subtraction.
Details of energy subtraction techniques in digital radiography and fluoroscopy are set forth in the following technical publications, all which are hereby incorporated specifically by reference:
Hall, A. L. et al: "Experimental System for Dual Energy Scanned Projection Radiology". Digital Radiography proc. of the SPIE 314: 155-159, 1981;
Summer, F. G. et al: "Abdominal Dual Energy Imaging". Digital Radiography proc. SPIE 314: 172-174, 1981;
Blank, N. et al: "Dual Energy Radiography: a Preliminary Study". Digital Radiography proc. SPIE 314: 181-182, 1981; and
Lehman, L. A. et al: "Generalized Image Combinations in Dual kVp Digital Radiography", Medical Physics 8: 659-667, 1981.
The above incorporated article by Lehman, et al describes more recently conceived techniques for modifying the above described simple subtraction technique to enhance the quality of the energy subtracted image.
It has been proposed in energy subtraction to utilize a particular type of dual energy detector assembly which can produce separate signals representing each of lower and higher x-ray energy incident on the dectector. Such a detector assembly enables the practice of energy subtraction without the necessity for switching kVp x-ray output levels, or employing other means for periodically attenuating the x-ray beam, such as rapid interposition and removal of a filter to and from the x-ray path. Such a detector employs a dual layer of phosphor-detector elements, wherein the phosphor material of a first, or front, layer preferentially responds to energy of a relatively lower energy value. A second, or rear, detector layer preferentially responds to x-ray energy in a higher range. Such a detector, and its method of use, is described in published European Patent Application No. 83307157.4 published on Aug. 8, 1984 by Gary T. Barnes, which published application is hereby expressly incorporated by reference.
Since dual energy techniques can produce material-specific images, wherein substantially only bone, or substantially only soft tissue, are imaged, calibration of the system is desirably performed for both bone and soft tissue, i.e., for both low and high energy response. In doing this, it is known to use portions of aluminum to simulate radiation attenuation characteristics of bone, and to use portions of acrylic to simulate attenuation characteristics of soft tissue.
Prior art calibration techniques include interposition in the x-ray path, between the source and the detector, of various combinations of thicknesses of acrylic and aluminum in sets of different thickness combinations, actuating the source and monitoring the system output in the presence of the various combinations to determine how the imaging system response depends on the various acrylic/aluminum thickness combinations.
Two basic approaches to dual energy calibration have been proposed. A sequence of images may be taken with different thickness combinations of acrylic and aluminum covering the entirety of each image. Another proposal has been that only a single image be taken, but that image contains, in a number of different discrete regions, representations of several different acrylic/aluminum thickness combinations. Also, use of various step wedges has been proposed, wherein one device includes several different thickness combinations, distributed among respective areas, for performing calibration in accord with the latter of the above indicated proposals. The portions of acrylic and aluminum define surfaces parallel to the plane of the detector.
One problem with the prior art calibration apparatus is that the distance the x-rays travel through the acrylic or aluminum varies slightly with the angle of the path of the x-rays with respect to the detector and calibration material. Regions of the image toward the boundary of the detector receive somewhat less radiation, because rays incident on those outer regions travel a somewhat longer distance through the attenuating calibration material. This phenomenon gives rise to an inaccurately nonuniform calibration result.
Where only one exposure is made, and different regions of the same image are attenuated with differing aluminum/acrylic thickness combinations, averaging techniques, useful in minimizing the effect of individual detector element nonuniformity in calibration, are less effective, inasmuch as only the detector elements corresponding to the portion of the image attenuated by a particular acrylic/aluminum thickness combination are susceptible of averaging.
Where different regions are attenuated differently in making a calibration image, edge effects of the radiation interacting with the various calibration elements sometimes adversely affect uniformity and accuracy of response and reliability of the calibration technique.
Also, a large number of calibration element thicknesses of aluminum and acrylic are required, in order to perform calibration at a relatively large number of calibration points for both soft tissue and bone attenuation.
Calibration elements must be very carefully machined so that their effect on radiation is precisely predictable. Where complex shapes, such as step wedges, are used for calibration elements, the cost of machining can be considerable.
Since it is desirable to calibrate with a large number of combinations of acrylic/aluminum thicknesses, calibration can be quite time consuming due to the need for manually placing, removing and replacing combinations of calibration elements in the x-ray beam.
It is an object of the present invention to provide a light weight, inexpensive and accurate calibration apparatus and method wherein all x-rays traverse paths of equal length in penetrating the apparatus.
The present invention will be more fully understood by reference to the following detailed description, and to the drawings, in which: